The invention relates to the hydrocracking of vacuum gas oil or various other typical hydrocracking feedstock oils or mixtures thereof.
In hydrocracking technology, reactor operating conditions are dictated either by product quality requirements or by catalyst life. It is impossible to optimize processing conditions in a single reactor because operating conditions in the reactor are set by the most difficult components of the feed. For example, the conditions in the reactor could be set by the amount of nitrogen in the feed. Typically, in the first reactor treating raw feed, conditions are severe (high-temperature) and not conducive to aromatic saturation. Moreover, once products are formed from hydrocracking reactions, they compete with the heaviest fractions of the feed (nominally 700xc2x0 F.+material) to gain access to the active catalyst sites. Occlusion of the products (700xc2x0 F.xe2x88x92material) from the active sites by the heavy products is very likely.
Consequently, for a given conversion level, single reactor systems operating at the same pressure levels as multi-reactor systems produce inferior quality products. In order to compensate for this shortfall in product quality, units are run at higher pressures and with lower space velocities. In most cases, there is considerable giveaway in product quality for at least one major product especially at start-of-run conditions, as operators select an operating pressure level to guarantee the quality of all products and extend the catalyst run length. For example, the hydrocracked Jet/Kerosene Smoke Point is often 30 mm at start-of-run when the specification requires 20 mm. Similarly, the hydrocracked Diesel Cetane Index is often around 60 when the required value is 50. This product quality giveaway translates to a waste of hydrogen. In most refineries, hydrogen is an expensive commodity.
The present invention relates to a hydrocracking and hydrotreating process which minimizes hydrogen consumption and reduces the overall reactor and catalyst volumes for a given level of performance for the production of diesel oil and lighter materials including kerosene and naphtha. The invention includes a first hydrogenation process in a main reactor employing a combination of cocurrent and countercurrent processing. The hydrocracking feedstock oils together with hydrogen are fed down through the top section of the main reactor containing a layered system of hydrotreating and hydrocracking catalysts for the cocurrent processing. Below the top section is a vapor/liquid disengaging zone where vapor and liquid are separated and the vapor containing the majority of the hydrogen sulfide and ammonia formed by hydrotreating and the majority of the diesel oil and lighter materials formed by hydrocracking are withdrawn. The separated liquid then flows down through the lower section also containing catalyst beds for the countercurrent processing. High purity make-up hydrogen is introduced up into the bottom of the reactor which flows up countercurrent to the downflowing liquid for further hydrogenation and for stripping the diesel and lighter materials present or formed in the lower countercurrent section. The bottoms containing primarily the uncracked heavy oil fraction together with some diesel and lighter materials and the separated vapors from the vapor/liquid disengaging zone are then processed in a post-treatment reactor to complete the hydrotreating and hydrocracking for final product quality and to facilitate the nitrogen and sulfur separation. The process using the cocurrent/countercurrent main reaction system with make up hydrogen addition at the bottom in combination with the post-treatment reaction system favors sulfur and nitrogen removal and aromatic saturation as well as final product yields and quality using lower reactor volume, lower catalyst volume, increased catalyst run length and lower hydrogen consumption.